1. Field of the Invention
This invention relates generally to processes for making sodium bicarbonate, and more particularly to a process for making sodium bicarbonate from Nahcolite-rich solutions by use of solubility suppression with addition of salt.
2. Brief Description of the Prior Art
Sodium bicarbonate (NaHCO.sub.3) is an important industrial chemical used in water and air pollution control, various industrial processes, as a fire extinguisher and in higher grades as an antacid, an agricultural feed additive, and as a baking powder and component of foods.
Typically, there are three processes which are used to produce or recover sodium bicarbonate: (1) the carbonization of naturally or synthetically produced sodium carbonate solutions; (2) crystalization of a naturally occurring or by-product sodium bicarbonate solution; or (3) carbonization of ammonium carbonate and reacting with sodium chloride.
In the process of carbonization of naturally produced sodium carbonate solutions, the sodium carbonate solution is usually a saturated brine solution containing a variety of sodium salts. The solubility of sodium bicarbonate is greatly depressed by the presence of sodium chloride, sodium sulfate or other salts.
There are several patents which disclose various processes for producing or recovering sodium bicarbonate.
Ueber et al, U.S. Pat. No. 3,759,328 discloses a process for recovering oil shale wherein hot water or steam is injected into the bore hole in an oil shale formation to expand the cavity and permeabilize and rubble the oil shale rock for recovery of shale oil through pyrolysis of the kerogen contained in the oil shale. Pressure is maintained above the decomposition pressure of the carbonate minerals (trona and Nahcolite) such that the minerals decompose and shale oil is recovered.
Beard et al, U.S. Pat. No. 3,779,602 discloses a pressure/temperature dependent process for solution mining sodium bicarbonate minerals from an oil shale formation in a Nahcolite zone by injecting steam at the top of a steam filled cavity at a temperature in excess of 250.degree. F., and maintaining the cavity temperature greater than 250.degree. F. to maximize cavity growth rate. Condensation of steam to a liquid form occurs upon contact with the formation and results in collection of superheated water in the lower portion of the cavity. The pressure is maintained at a pressure at which the sodium-carrying capacity of the superheated water is at a maximum and below which there will be excess thermal decomposition of bicarbonate to carbonate and precipitation of carbonate.
Towell et al, U.S. Pat. No. 3,792,902 discloses a process for solution mining of trona or Nahcolite wherein hot water of low alkalinity is injected into the base of a production tubing string or adjacent the intake to prevent mineral precipitation and plugging of the production well by dilution. The process utilizes hot water or steam (325.degree. F.) and a pressure of 1500 psi to produce a mixed Na.sub.2 CO.sub.3 /NaHCO.sub.3 -rich production solution.
Beard, U.S. Pat. No. 3,759,574 discloses a process for producing oil shale from a trona and/or Nahcolite formation which includes initial permeabilization of the formation by the dissolution of the sodium minerals with a hot aqueous solution.
Kelmar, U.S. Pat. No. 4,375,302 discloses a process for recovering multi-minerals from oil shale wherein a NaOH solution is injected into the oil shale to dissolve NaHCO.sub.3 and convert it to an Na.sub.2 CO.sub.3 solution in order to develop porosity in the oil shale in preparation for recovery of shale oil.
Rosar et al, U.S. Pat. No. 4,815,790 discloses a Nahcolite solution mining process wherein a hot barren aqueous liquor at a temperature of from about 85.degree. F. to about 300.degree. F. is injected at a pressure of below about 150 psig and circulated in a cavity in a Nahcolite (NaHCO.sub.3) bed for a time sufficient to produce a pregnant liquor having an increase in the concentration of NaHCO.sub.3 in the range of from about 3% to about 20% while maintaining sodium carbonate (Na.sub.2 CO.sub.3) concentration in the range of 0.25% to 4% (preferably less than 2.5%). The pregnant liquor is withdrawn and cooled to about 25.degree. F. to 120.degree. F. to precipitate the sodium bicarbonate (NaHCO.sub.3) by crystalization and to produce a barren liquor which may be reheated and reinjected.
The cooling/crystalization process taught by Rosar et al is greatly influenced by the presence of impurities such as sodium carbonate (Na.sub.2 CO.sub.3) and salt (NaCl) and requires that the salt (NaCl) concentration be maintained at below about 6% (preferably below about 1.0%).
The present invention is distinguished over the prior art in general, and these patents in particular by a process for making sodium bicarbonate from Nahcolite-rich solutions which utilizes solubility suppression with addition of salt. Hot water pumped into a Nahcolite-rich formation, which may contain salt (NaCl) and other impurities in excess of 6% by weight, dissolves the Nahcolite (NaHCO.sub.3) and salt. The dissolved Nahcolite is brought to the surface as a pregnant brine liquor containing Nahcolite, salt, and other impurities, which is then de-gassed under pressure. The pregnant liquor is then mixed with a salt slurry and its temperature reduced to saturate it with salt and force precipitation of Nahcolite out of the pregnant liquor. Carbon dioxide (CO.sub.2) is injected while mixing the pregnant liquor with the salt to facilitate precipitation and conversion of any sodium carbonate (Na.sub.2 CO.sub.3) present into sodium bicarbonate (NaHCO.sub.3). The sodium bicarbonate (NaHCO.sub.3) is dewatered and filtered to form a sodium bicarbonate (NaHCO.sub.3) filter cake, which is washed with fresh cold water to remove salt (NaCl), sodium carbonate (Na.sub.2 CO.sub.3), and other impurities as a salt-rich brine filtrate, and the filter cake is dried to produce a high grade of natural sodium bicarbonate (NaHCO.sub.3). The filtrate may be processed to produce hot water condensate which can then be used to dissolve the Nahcolite, and the salt slurry produced can be used to saturate the pregnant liquor and force Nahcolite precipitation, and a portion may be dried and used as a saleable by-product.
It should be noted that in the process in accordance with the present invention, the presence of salt will not be deleterious to the processing of the pregnant liquor for recovery of the sodium bicarbonate and is not influenced by the purity of the pregnant liquor and can be used to recover Nahcolite from salt contaminated formations or surface processing facilities where the pregnant liquor contains salt in excess of 6%.